This invention relates to a method of preparing improved eutectic and hyper-eutectic alloys and metal matrix composites containing such alloys.
Metal matrix composite materials have gained increasing acceptance as structural materials. Such composites typically are composed of reinforcing particles, such as fibres, grit, powder or the like that are embedded within a metallic matrix. The reinforcement imparts strength, stiffness and other desirable properties to the composite, while the matrix protects the fibres and transfers load within the composite. The two components, matrix and reinforcement, thus cooperate to achieve results which are improved over what either could provide on its own. A typical composite is an aluminum alloy reinforced with particles of silicon carbide or alumina.
A major difficulty in the production of good quality metal matrix composites is segregation of the reinforcing particles. The segregation of particles occurs in the liquid state as well as during solidification. The segregation in the liquid state can be overcome by a proper mixing of the liquid. However, even if the particles are uniformly distributed in the liquid state, they may still segregate during solidification. When metal matrix composites are in the process of solidifying, the reinforcing particles can be rejected ahead of the solidification interface, and may agglomerate in the interdendritic liquid which solidifies last. For instance, in aluminum matrix composites, solid .alpha.-aluminum dendrites are formed and the reinforcing particles are pushed ahead of the growing dendrites to be finally trapped in the last to solidify interdendritic liquid. The reinforcing particles are not found inside the aluminum dendrites and, in this sense, it can be said that the aluminum dendrites do not "wet" the reinforcing particles. This results in a highly inhomogeneous distribution of reinforcing particles in the as-cast materials.
Whether reinforcement particles are pushed by the solidification interface or are engulfed is primarily dependent upon the degree of wetting between the particles and the solid surface. If the solid surface wets the particles, they are engulfed by the solid surface. In this case the particle distribution in the solidified material is as uniform as it was in the liquid state. On the other hand, if the solid surface, e.g. aluminum dendrite surface, does not wet the particles, they are pushed away, resulting in interdendritic segregation.
In certain alloy systems, such as eutectic or hyper-eutectic systems, intermetallic compounds may precipitate directly from a melt of the alloy. These intermetallic compounds often tend to be coarse, brittle particles, and these particles tend to segregate due to density difference, particularly when the solidification rate is slow.
There is some evidence in the prior art of a degree of wetting between refractory particles and intermetallic surfaces. For instance P. K. Rohatgi, "Interfaces in Metal Matrix Composites", p. 185, The Metallurgical Society/AIME, New Orleans, 2-6 Mar. 1986, has shown an example of primary NiAl.sub.3 nucleating on graphite particles during the solidification of a hyper-eutectic Al-Ni alloy. He also noted that there is a tendency for primary Si to nucleate on graphite and alumina particles during the solidification of a hyper-eutectic Al-Si alloy.
Solidification studies of grain refining Al-Ti-B alloys are described in K. Kuisalaas and L. Backerud, Solidification Process 1987, p. 137, Institute of Metals, Sheffield, U.K., 21-24 Sep. 1987. These studies noted that TiAl.sub.3 intermetallics tended to adhere to the surface of TiB.sub.2 particles.
A study on aluminum alloys for elevation temperature applications is described in D. A. Granger et al., "Aluminum Alloys for Elevated Temperature Applications" p. 777-778, AFS Transactions, 86-143. Traditionally, casting alloys for elevated temperature applications were made by adding large amounts of Cu or Ni, e.g. up to about 8 wt % Cu and 5.5 wt % Ni. It has been generally understood that high volume fractions of the intermetallics so formed improve the high temperature properties. However, the amount of these elements which could be added was restricted because they formed large brittle intermetallic primaries on solidification if the addition was beyond a certain limit. The amount of Mn that could be added was limited to less than 0.5 wt %.
It is the object of the present invention to provide a technique for improving eutectic and hyper-eutectic alloys and for solving the problem of the segregation of the reinforcement particles in metal matrix composites made from eutectic or hyper-eutectic alloys which tends to occur during solidification. It is a further object of the invention to produce new alloy products having improved high temperature properties.